Weird glass artifact
Ok. The solution was to recalculate normals by using Shift + N . The problem was, that faces were facing wrong way. Blender renders fronts of faces in higher quality than backs, and mine were facing front and back in checkerboard, so that explains checkerboard thing. Recalculating normals rotated  them the right way. I think that they rotated when I was messing with UV unwrapping.